1. Field of the Invention
The present invention relates to a connecting structure for an intake tube, particularly an intake manifold, which is designed to introduce air from an air cleaner to a cylinder head of an internal combustion engine. More specifically, the present invention relates to a structure for connecting an intake tube to an intake port of a cylinder head.
2. Description of the Related Art
Conventionally, an intake manifold used as a main part of an intake apparatus has been typically made of cast iron or an aluminum alloy for attachment to a cylinder head by bolting. Recently, however, an intake manifold made of a synthetic resin material is used for realizing a reduction in cost and weight, as disclosed in JP-U-2-64745 for example.
Specifically, the prior art intake manifold disclosed in the above Japanese document includes a main tube for connection to an air cleaner, and a plurality of branch tubes each branching from the main tube for connection to a corresponding intake port of a cylinder head. For gas tightness, a seal ring having a circular cross section is fitted around a plug end portion of each branch tube for insertion into a socket end portion of the corresponding intake port together with the branch tube.
In assembly, the seal ring is compressively deformed as a whole from a circular cross section to an ellipsoidal cross section for insertion into the intake port, and the degree of such deformation must be set relatively high for providing reliable air tightness because the internal pressure within the intake port varies considerably during the engine operation. Thus, it is rather troublesome to insert the plug end portion of each branch tube into the intake port under large deformation of the seal ring. Further, the seal ring is likely to be frictionally damaged due to its insertion under large deformation.
On the other hand, the seal ring serves to prevent gas leakage not only from the exterior to the interior but also from the interior to the exterior because the internal pressure within the intake port varies between a subatmospheric level to a superatmospheric level. Thus, if the elastic deformation of the seal ring is adjusted to completely resist gas leakage from the exterior to the interior, the seal ring will also resist gas leakage from the interior to the exterior. As a result, even when the internal pressure within the intake port rises abnormally, the seal does not permit partial gas escape, so that the intake manifold and/or the air cleaner connected thereto may be broken due to an abnormal pressure rise.